1. Field of the Invention
The present invention relates to an image processing device, an image processing program and an image processing method for performing color conversion on video signals.
2. Description of the Related Art
An image pickup device, which has a color filter for primary colors or complementary colors on the front face, is used for many of image pickup systems in currently manufactured digital cameras, video cameras and the like. Video signals acquired from the image pickup device are subjected to several processings such as color conversion processing, gradation correction processing and edge enhancement processing, and then outputted as a final image.
Meanwhile, several types of color filters, each of which has a different spectral characteristic, are used for image pickup systems, and additionally, image pickup devices themselves have different spectral characteristics for each model. Accordingly, when more than one image pickup system having a different characteristic of color is used, each of the image pickup systems may output a final image having different color reproduction with one another even if the image pickup systems have an identical circuit to handle video signals.
As a technique to solve such a problem, a technique of a color management system (hereinafter referred to as CMS) has been proposed and adoption of the technique is necessary. In the CMS, color conversion processing using a matrix operation or an LUT is usually performed to match color signals. When the color conversion processing is performed using an LUT, there is the advantage of achieving high-precision processing, but large memory capacity is needed, thus resulting in enlargement of hardware scale.
On the other hand, when the color conversion processing is performed using a matrix operation, there is the advantage that memory capacity can be substantially reduced, thus resulting in reducing the hardware scale even though the precision of color conversion is somewhat inferior to that of the LUT.
Thus, as a practical approach, color conversion processing using a matrix has been widely utilized on product development.
However, the technique using the CMS described above may not achieve favorable color reproduction in a low color region, in particular. For example, the color conversion of the CMS using a matrix operation blends every color evenly. This allows each chromatic color to favorably reproduce their colors evenly, that is, precision in color conversion processing on the chromatic colors is kept. However, with such a technique, the signal of a color that should be originally achromatic is sometimes converted into a chromatic color signal, thus precision in the color reproduction on the achromatic region may be sacrificed.
Since a human visual characteristic is more sensitive to noise in the achromatic region than in the chromatic region, matrix conversion using a large coefficient has the adverse effect of increasing noise, which is not desired.
In order to solve such problems with the CMS, a technique for performing specific color conversion processing on the achromatic region has been proposed.
For example, Japanese Patent Application Laid-Open Publication No. 8-79550 discloses a technique, which determines whether or not input image data belongs to an achromatic neighboring range, and performs, on the image data determined to belong to the achromatic neighboring range, color adjustment appropriate to the achromatic neighboring range, or performs, on the image data determined to belong to a region other than the achromatic neighboring range, color adjustment to achieve an appropriate common chromatic color.
Further, Japanese Patent Application Laid-Open Publication No. 9-27915 discloses a technique, which determines whether or not a color prior to color conversion is achromatic, and removes chromatic components after color conversion if the color is determined to be achromatic, or uses the result of a matrix-based color conversion processing if the color is determined to be not achromatic.
As described above, the techniques described in Japanese Patent Application Laid-Open Publications No. 8-79550 and No. 9-27915 perform independent and appropriate color conversion processing on an achromatic color and a chromatic color, respectively, so as to perform high-precision color conversion processing on both the achromatic color and chromatic color.
Meanwhile, Japanese Patent Application Laid-Open Publications No. 2004-72422 and No. 2004-266323 describe a technique for estimating the amount of noise included in a video signal.
An object of the present invention is to provide an image processing device, an image processing program and an image processing method capable of performing high-quality and high-precision color conversion processing according to whether or not a video signal belongs to an achromatic region.